A showcase is actually just a gallery with a different classification.
To create a showcase you'll need first to create a gallery. Then use the toggle switch in the gallery details editor to make your gallery a showcase.
Showcases are for showing off edited selections of your best work in a particular area of specialisation - like weddings, documentary, editorial and so on.
You can only create one showcase for each area of specialisation.
Specialisations can be selected from the drop down list which appears on your profile page and at the bottom of the gallery details editor when you toggle to make your gallery a showcase.